Guide To The Western Interior Sea

Between 100 and 70 million years ago during the Cretaceous period, the part of North America we now call the Great Plains was in the process of sinking while sea levels rose to some of the highest levels ever seen in Earth’s history. The result of these two processes working in tandem was the slow ingression of the waters of the Arctic Ocean and the Gulf of Mexico into the American midcontinental region, with inland seas forming over the continental shelf in the North and South. Eventually these northern and southern seas joined together, forming a long narrow body of water we now call the Western Interior Sea. At its height, this sea spanned from southern Texas to the Canadian Arctic, splitting the continent in two. The waters of this seaway were warm and shallow, estimated to have been no more than 3,000 feet deep. Today we find the fossils of the ancient denizens of these prehistoric waters in the layers of limestone and shale located beneath much of the Great Plains, including the remains of mosasaurs, plesiosaurs, giant fish, sharks and even some rare fossils of dinosaurs whose bodies were washed out to sea.